Electricity and steam generation from a helium-cooled nuclear reactor

ABSTRACT

Disclosed herein is a method comprising heating helium in a core of a nuclear reactor; extracting heat from the helium; superheating water to steam using the heat extracted from the helium, expanding the helium in a turbine; wherein the turbine is in operative communication with an electrical generator; and generating electricity in the electrical generator.

This application is a divisional application of application Ser. No.11/313,502, filed Dec. 21, 2005.

BACKGROUND

This disclosure relates to the generation of electricity and steam. Inparticular, this disclosure relates to the generation of electricity andsteam from a helium-cooled high temperature nuclear reactor by means ofa closed helium Brayton cycle and a heat recovery steam generator.

Fossil fuel combustion has been identified as a significant contributorto numerous adverse environmental effects. For example, poor local airquality, regional acidification of rainfall that extends into lakes andrivers, and a global increase in atmospheric concentrations ofgreenhouse gases (GHG), have all been associated with the combustion offossil fuels. In particular, increased concentrations of GHG's are asignificant concern since the increased concentrations may cause achange in global temperature, thereby potentially contributing to globalclimatic disruption. Further, GHG's may remain in the earth's atmospherefor up to several hundred years.

One problem associated with the use of fossil fuel is that theconsumption of fossil fuel correlates closely with economic andpopulation growth. Therefore, as economies and populations continue toincrease worldwide, substantial increases in the concentration of GHG'sin the atmosphere are expected. A further problem associated with theuse of fossil fuels is related to the inequitable geographicaldistribution of global petroleum resources. In particular, manyindustrialized economies are deficient in domestic supplies ofpetroleum, which forces these economies to import steadily increasingquantities of crude oil in order to meet the domestic demand forpetroleum derived fuels.

Nuclear reactors do not emit appreciable pollutants or GHG's into theatmosphere and can provide energy independence to economies that aredeficient in fossil fuels. The majority of today's nuclear reactors arewater-cooled and generate electricity through steam generation andsubsequent expansion through a steam turbine. Because of the relativelylow temperature steam produced by these reactors (generally below 300°C.), the net thermal efficiency for electrical generation is relativelylow (generally below 35%). An additional shortcoming of these reactorsis that the steam produced is too cold for many potential industrialapplications, such as hydrogen production by steam methane reforming(SMR) of natural gas or hydrogen production by solid oxide electrolysisof steam. Intermediate temperature solid oxide electrolyzer systemsgenerally operate at temperatures of about 700 to about 900° C. Steamundergoes electrolysis in the cathode side of a solid oxide electrolyzercell to generate hydrogen. Electrical energy is required to electrolyzethe steam, so it is desirable to have a nuclear reactor system that canproduce high-temperature steam as well as electrical energy.

Graphite-moderated nuclear reactors that are cooled with helium gas canachieve very high helium exit temperatures, from 700° C. to potentially1,000° C. Many systems have been proposed for the production ofelectrical energy and high-temperature steam using helium-cooledreactors.

Systems have been proposed that indirectly couple a steam Rankine cycleto the primary helium coolant loop. High pressure steam is generated ina boiler heated by the helium used to cool the primary loop. The highpressure steam is partially expanded through a steam turbine to produceelectrical energy. A portion of the partially expanded steam is thenreheated through a second heat exchanger heated by primary loop helium.This intermediate pressure reheated steam can then be used forapplications such as solid oxide electrolysis. This type of system has arisk of steam ingress into the nuclear core due to the high-pressuresteam generators, where the steam can be at a higher pressure than theprimary helium coolant. Steam ingress into the core is undesirablebecause it can corrode the graphite moderator and graphite-coated fuel,and can also cause a reactivity insertion due to the moderating effectof steam.

Other systems have been proposed that indirectly couple a Braytontopping cycle to the primary helium coolant loop and further indirectlycouple a steam Rankine bottoming cycle to the Brayton cycle, in aconcept known generally as an indirect combined cycle. Heat istransferred through an intermediate heat exchanger to a Brayton cycleemploying a compressed gaseous working fluid, such as air or helium.This heated gas is expanded through a turbine to produce electricity.Expanded gas then passes through a heat recovery steam generator toproduce steam, which can be expanded through a steam turbine foradditional electrical production or alternatively can be used forindustrial applications. This system does not produce steam with therequired high temperature for solid oxide electrolysis, however.Furthermore, this system requires the use of a very large and expensivegas-to-gas intermediate heat exchanger.

Other systems have been proposed that directly expand the helium througha turbine to produce electricity using a direct Brayton cycle. Toproduce steam in addition to electricity, systems have been proposedthat divert a fraction of the helium coolant exiting the nuclear core toa second loop in parallel with the Brayton cycle loop. Helium in thissecond parallel loop generates steam in a steam generator. Such systemshave several undesirable features—they do not efficiently use the highenergy available in the high-temperature helium in the second parallelloop and they use a second compressor in the second parallel loop. Inaddition, they use a large and expensive gas-to-gas recuperator totransfer heat from the turbine exhaust to the reactor inlet forefficient electrical generation.

It is therefore desirable to have a system that produces bothelectricity and low-pressure steam using a helium-cooled nuclear reactorin an economical and safe manner.

SUMMARY

Disclosed herein is a method comprising heating helium in a core of anuclear reactor; extracting heat from the helium; superheating water tosteam using the heat extracted from the helium; expanding the helium ina turbine; wherein the turbine is in operative communication with anelectrical generator; and generating electricity in the electricalgenerator.

Disclosed herein too is a system for producing electricity and steamcomprising a Brayton power conversion cycle employing helium as theworking fluid in a first closed loop wherein the first closed loopcomprises a heat source comprising a nuclear reactor; wherein the coreof the nuclear reactor is cooled using helium; a power conversion systemcomprising a turbine, a compressor, and an electrical generator, whereinthe compressor, the turbine, and the electrical generator are inoperative communication with each other; wherein the turbine is locateddownstream from the heat source and is in fluid communication with theheat source; a heat recovery steam generator, located downstream of theturbine and in fluid communication with the turbine, where the steamgenerated is at a pressure that is less than or equal to about thepressure of the helium in the first closed loop; and a waste heatremoval heat exchanger, located downstream of the heat recovery steamgenerator and in fluid communication with the heat recovery steamgenerator.

DETAILED DESCRIPTION OF FIGURES

FIG. 1 is a depiction of one exemplary embodiment of a system that cangenerate electricity and steam using helium in a Brayton powerconversion cycle with a heat recovery steam generator;

FIG. 2 is a depiction of one exemplary embodiment of a self-sufficientsystem for generating hydrogen, wherein the electricity and steam needsof the system are met by the system itself, without the importation orexportation of electricity or steam to or from an external sourcerespectively; and

FIG. 3 is a depiction of one exemplary embodiment of a self-sufficientsystem for generating hydrogen; wherein the pressure, temperature andmass flow at various points in the system are shown in the Tables 1 and2.

DETAILED DESCRIPTION

It is to be noted that the terms “first,” “second,” and the like as usedherein do not denote any order, quantity, or importance, but rather areused to distinguish one element from another. The terms “a” and “an” donot denote a limitation of quantity, but rather denote the presence ofat least one of the referenced item. The modifier “about” used inconnection with a quantity is inclusive of the stated value and has themeaning dictated by the context (e.g., includes the degree of errorassociated with measurement of the particular quantity). It is to benoted that all ranges disclosed within this specification are inclusiveand are independently combinable.

Furthermore, in describing the arrangement of components in embodimentsof the present disclosure, the terms “upstream” and “downstream” areused. These terms have their ordinary meaning. For example, an“upstream” device as used herein refers to a device producing a fluidoutput stream that is fed to a “downstream” device. Moreover, the“downstream” device is the device receiving the output from the“upstream” device. However, it will be apparent to those skilled in theart that a device may be both “upstream” and “downstream” of the samedevice in certain configurations, e.g., a system comprising a recycleloop.

Disclosed herein is a system that produces electricity and low-pressuresteam from a helium-cooled high temperature nuclear reactor by using aclosed helium Brayton cycle and a heat recovery steam generator (HRSG).In one embodiment, electricity and steam are produced for use in a solidoxide electrolyzer system that is in operative communication with ahelium cooled nuclear reactor. This system produces steam andelectricity in the proper proportion for use by a solid oxideelectrolysis cell or cells for the production of hydrogen, such that thesystem does not export or import additional steam or electricity to orfrom external sources.

In one embodiment, the system uses helium as a medium for transferringheat (generated in a nuclear core) to steam that can optionally beelectrolyzed into hydrogen and oxygen in a solid oxide electrolyzercell. The helium is used to cool the core of a high temperature nuclearreactor. The heat extracted by the helium in the process of cooling thecore is then used to drive a gas turbine that operates using a directBrayton power conversion cycle. The gas turbine is in operativecommunication with a generator that generates electricity. The hothelium is also advantageously used to generate steam that can beoptionally used for the generation of hydrogen. The system isadvantageous since there is a minimization of the possibility of wateringress into the core of the nuclear reactor because the water and steamat all points in the system are at an equal or lower pressure than thehelium coolant. Further, the helium coolant is a single phase coolantthat is inert and has minimal reactivity effects.

With reference now to FIG. 1, the system 10 comprises a first closedloop comprising a heat source 102. The heat source 102 comprises anuclear reactor that employs helium as a coolant. The heat source 102 isin operative communication with a power conversion system comprising aturbine 106, a compressor 120, and an electrical generator 126 thatoperates on a Brayton cycle. As can be seen in the FIG. 1, the turbine106 is located downstream from the heat source 102 and is in fluidcommunication with the heat source 102. A heat recovery steam generator(HRSG) 112 is located downstream of the turbine 106 and is in fluidcommunication with the turbine 106, where the steam generated is at apressure that is less than or equal to about the pressure of the heliumin the first closed loop. The system 10 further comprises a waste heatremoval heat exchanger 118, located downstream of the HRSG 112 and influid communication with the HRSG 112.

In one embodiment, the heat source 102 is a nuclear reactor that employsmachined graphite blocks as the moderator and as the core structuralelement. Coated fuel particles containing fissile material are compactedinto cylindrical pellets and inserted into holes drilled into thegraphite blocks. Helium coolant flows through additional holes drilledthrough the graphite blocks. In another embodiment, the heat source is anuclear reactor that employs coated fuel particles containing fissilematerial that are compacted into pebbles. These pebbles are thenassembled to form a “pebble bed” comprising the core of the reactor.Helium coolant flows between the pebbles.

In one embodiment, the power conversion system comprises a turbine 106,an electric generator 126, and a compressor 120 on a common horizontalshaft. In another embodiment, the power conversion system comprises aturbine, an electric generator, and compressors on a common verticalshaft. In one embodiment, the electrical generator may be located at thecompressor end of the shaft, as shown in FIG. 1. In another embodiment,the electrical generator may be located at the turbine end of the shaft.In one embodiment, the generator is directly coupled to the shaft androtates synchronously with the turbomachine. In another embodiment, thegenerator is coupled to the shaft via a gearbox that reduces the speedof rotation of the generator relative to the turbomachine shaft.

The HRSG 112 is operative to extract heat from the helium in the firstloop and transfer it to water to convert water into steam. In oneembodiment, the HRSG 112 is a shell-and-tube heat exchanger, where hothelium passes over tubes filled with water or steam. In one embodiment,the tubes are coiled. In another embodiment, the tubes have exteriorfins attached to improve heat transfer. In an exemplary embodiment, theHRSG 112 is segmented into three sections. Referring now to FIG. 2, theHRSG 112 is segmented into three sections: the steam superheater 110,the steam evaporator 113, and the economizer 114. The steam superheater110 transfers heat from hot helium to saturated steam generated in theevaporator, and heats the steam to a temperature above its saturationtemperature to become superheated steam. The steam evaporator 113transfers heat from hot helium to saturated liquid water from theeconomizer, and heats the water causing it to boil and become saturatedsteam. The economizer 114 transfers heat from hot helium to liquid waterand heats the water up to its saturation temperature.

Referring again to FIG. 1, the waste heat removal heat exchanger 118 isoperative to remove waste heat from the helium prior to compression inthe compressor 120. In one embodiment, the waste heat removal heatexchanger 118 is a shell-and-tube heat exchanger employing water as acooling fluid. In one embodiment, this cooling water is extracted from asource of cool water such as a lake and is returned to this source afterflowing through the waste heat removal heat exchanger. In anotherembodiment, the cooling water is circulated in a closed loop and ispumped through a cooling tower for ultimate rejection of the waste heatto the atmosphere. In another embodiment, the waste heat removal heatexchanger 118 utilizes forced-circulation air as a cooling fluid.

As detailed in FIG. 2, the system 10 can comprise additional optionalfeatures that can be used for the optional generation of hydrogen, ifdesired. The first loop 100 is in operative communication with a secondloop 200 that optionally comprises the solid oxide electrolyzer cell202. In an alternative embodiment, the second loop 200 may comprise asteam methane reformer in lieu of the solid oxide electrolyzer cell 202.In yet another alternative embodiment, the second loop 200 may comprisea radiator in lieu of the steam methane reformer or the solid oxideelectrolyzer cell 202. In one embodiment, the solid oxide electrolyzercell 202 is an intermediate temperature operating cell that functions ata temperature of about 700 to about 850° C. The solid oxide electrolyzercell 202 may be tubular or planar in assembly. The solid oxideelectrolyzer cell 202 is partitioned into an anode side and a cathodeside by a hermetic membrane comprising a solid oxide electrolyte.Alternating-current (AC) electrical power is converted into directcurrent (DC) electric power by an AC-DC converter, and the directcurrent electric power is supplied to the solid oxide electrolyzer cell202. The electrical energy facilitates the conversion (electrolysis) ofthe high-temperature steam supplied to the cathode side into molecularhydrogen and negative oxygen ions. Oxygen ions pass through the solidoxide electrolyte to the anode, where they combine to form molecularoxygen.

In one embodiment, the solid oxide electrolyzer cell 202 uses anelectrolyte that comprises yttria-stabilized-zirconia (YSZ),gadolinia-doped-ceria, samaria-doped-ceria, orlanthanum-strontium-gallium-magnesium oxide. Suitable anode materialsinclude mixed-ionic-electronic-conducting (MIEC) ceramics such aslanthanum-strontium-ferrite, lanthanum-strontium-cobaltite, orlanthanum-strontium-cobaltite-ferrite, and their combinations with anelectrolyte material such as those listed above.

In an example according to this embodiment, the solid oxide electrolyzercell 202 can further comprise an ion-conducting barrier layer toseparate the anode from the electrolyte. For example, a suitable barrierlayer that can be used between a YSZ electrolyte and alanthanum-strontium-cobaltite-ferrite includes samaria-doped-ceria andgadolinia-doped-ceria. Suitable cathode materials include the compositeNi/YSZ. In one embodiment, the Ni/YSZ is used at the operatingtemperature. In an example according to this embodiment, the solid oxideelectrolyzer cell 202 can further comprise a reducing environmentmaintained on the cathode side. For example, maintaining hydrogen in thesteam feed of at least about 5 mole percent can provide a reducingenvironment on the cathode side.

In another embodiment, the solid oxide electrolyzer cell 202 uses anelectrolyte-supported design. In one embodiment, the thickness of theelectrolyte is about 10 micrometers to about 400 micrometers, morespecifically about 25 micrometers to about 300 micrometers, mostspecifically about 50 micrometers to about 200 micrometers. Theelectrolyte can be fabricated by tape-casting, pressing, extruding,slip-casting, tape-calendaring, sintering, or the like, or a combinationcomprising at least one of the foregoing. The thickness of the cathodeand anode are each independently about 1 micrometer to about 200micrometers, more specifically about 5 micrometers to about 100micrometers, most specifically about 10 micrometers to about 50micrometers. The electrodes can be fabricated by screen printing, wetparticle spraying, tape-calendaring, tape-casting, sintering, or thelike, or a combination comprising at least one of the foregoing.

In another embodiment, the solid oxide electrolyzer cell 202 uses acathode-supported design. In this embodiment, the thickness of thecathode is about 25 micrometers to about 2000 micrometers, morespecifically about 50 micrometers to about 1000 micrometers, mostspecifically about 200 micrometers to about 500 micrometers.

The cathode can be fabricated by tape-casting, pressing ortape-calendaring and sintering.

The thickness of the electrolyte can be about 1 micrometer to about 100micrometers, more specifically about 2 micrometers to about 50micrometers, and most specifically about 5 micrometers to about 15micrometers. The electrolyte can be fabricated by tape-casting,tape-calendaring, screen-printing, or wet particle spraying andsintering. In some cases, the cathode and electrolyte are co-sintered.

The thickness of the anode can be about 2 micrometers to about 200micrometers, more specifically about 5 micrometers to about 100micrometers, most specifically about 10 micrometers to about 50micrometers. The anode can be fabricated by pressing, screen printing,wet particle spraying, tape-calendaring, tape-casting, sintering, or thelike, or a combination comprising at least one of the foregoing.

As noted above, the second loop 200 may comprise a steam methanereformer or a radiator (not shown) in lieu of the solid oxideelectrolyzer cell 202. The steam methane reformer is located downstreamof the heat recovery steam generator 112 and is in fluid communicationwith the heat recovery steam generator. The steam methane reformer isoperative to produce hydrogen from natural gas.

In another embodiment, the steam radiator is located downstream of theheat recovery steam generator 112 and in fluid communication with theheat recovery steam generator 112. The steam radiator is operative toheat a building.

In one embodiment, the first loop 100 is in operative communication withthe second loop 200 via a steam superheater 104. The steam superheateris upstream of the turbine 106 and is in fluid communication withturbine. In one embodiment, the steam superheater 104 can be ashell-and-tube type heat exchanger that facilitates the transfer of heatfrom hot helium to the steam present inside the tubes. In anotherembodiment, the steam superheater can be a plate-fin type heat exchangerthat facilitates the transfer of heat from hot helium on one side of theplates to the steam to steam present on the other side of the plates.

In one embodiment, the first loop 100 comprises a recuperator 116 thatis operative to transfer heat from the hot helium exiting the HRSG 112to the cold helium exiting the compressor 124. The hot side of therecuperator 116 is located downstream of the HRSG 112 and is in fluidcommunication of the HRSG 112. The cold side of the recuperator islocated downstream of the compressor 124 and is in fluid communicationwith the compressor 124. In one embodiment, the recuperator 116 is agas-to-gas heat exchanger. In an exemplary embodiment, the recuperator116 is a brazed plate-fin type heat exchanger.

In one embodiment, the first loop 100 comprises an intercooler 122 thatis operative to cool helium exiting the low-pressure compressor 120prior to further compression in the high-pressure compressor 124. In oneembodiment, the intercooler 122 is a shell-and-tube type heat exchanger,with cooling water passing through the tubes to extract heat from thehelium.

Helium exiting the nuclear reactor heat source 102 generally has apressure of about 40 to about 90 kg/cm² and a temperature of about 700to about 1,000° C. In one embodiment, the helium exits the nuclearreactor heat source at a pressure of about 50 to about 75 kg/cm² and atemperature of about 800 to about 950° C. In an exemplary embodiment,the helium exits the nuclear reactor heat source at a pressure of about52 to about 57 kg/cm² and a temperature of about 825 to about 875° C. Ascan be seen from FIG. 1, the hot helium is transferred to the turbine106 and the HRSG 112. If a superheater 104 is included upstream of theturbine 106 as in the FIG. 2, then hot helium is first transferred tothe superheater 104 prior to being transferred to the turbine 106. Thehot helium is used to heat steam to a temperature of about 400 to about900° C. The pressure of the steam in the second loop is lower than orequal to the pressure of the helium in the first closed loop.

The helium exits the superheater 104 at a temperature of about 600 toabout 900° C. and at a pressure of about 40 to about 90 kg/cm². Anexemplary temperature for the helium exiting the superheater 104 isabout 750 to about 850° C. and an exemplary pressure is about 50 kg/cm²to about 60 kg/cm². Helium is transferred to the high-pressure turbine104 from the first superheater 106. The high-pressure turbine 106 is inmechanical communication with a low-pressure gas compressor, ahigh-pressure gas compressor and an electrical generator. Heated heliumfrom the reactor expands through the turbine to drive the generator andgas compressors. The helium pressure decreases from about 50 to about 90kg/cm² prior to entering the turbine 106 to about 4 to about 12 kg/cm²after exiting the turbine 106. The temperature drops from about 600 toabout 900° C. prior to entering the gas turbine 106 to about 200 toabout 500° C., after exiting the turbine 106. An exemplary pressure forthe helium exiting the turbine is about 6 to about 9 kg/cm² and anexemplary temperature is about 250 to about 300° C.

The helium after the expansion is transferred to the HRSG 112. Heat fromthe helium is extracted in the HRSG 112 and is used to generate steamthat is optionally used for the generation of hydrogen in the solidoxide electrolyzer cell 202. As noted above, in an exemplary embodiment,the HRSG 112 comprises a steam superheater 110, an evaporator 113 and aneconomizer 114. The temperature of the helium is further reduced in theprocess of transferring its heat to the steam generated in the HRSG 112.The helium temperature generally decreases to about 100 to about 300° C.at the exit point of the HRSG 112, while the pressure of the helium alsodecreases slightly. An exemplary temperature for helium exiting the heatexchanger 108 is about 200 to about 225° C.

In one embodiment, helium exiting the HRSG 112 is then transferred tothe hot side of a recuperator 116. The temperature of the helium exitingthe hot side of the recuperator is decreased to about 100 to about 300°C.

The helium is then transferred to the waste heat removal heat exchanger118. In one embodiment, the temperature of the helium exiting the wasteheat removal heat exchanger is decreased to about 20 to about 75° C. Inan exemplary embodiment, the temperature of the helium exiting the wasteheat removal heat exchanger is decreased to about 25 to about 40° C.

In one embodiment, the helium is then transferred to a low-pressurecompressor 120. The low-pressure compressor compresses the helium to apressure of about 15 to about 30 kg/cm², increasing the heliumtemperature to about 150 to about 250° C. In an exemplary embodiment,the helium exits the low-pressure compressor at a pressure of about 18to about 22 kg/cm² and a temperature of about 190 to about 220° C.

In one embodiment, the helium is then transferred to an intercoolingheat exchanger 122. Helium exiting the intercooling heat exchanger 122is cooled to about 25 to about 75° C. In an exemplary embodiment, heliumexiting the intercooling heat exchanger 122 is cooled to about 30 toabout 45° C.

In one embodiment, the helium is then transferred to a high-pressurecompressor 124. The high-pressure compressor further compresses thehelium to a pressure of about 40 to about 90 kg/cm², increasing thehelium temperature to about 150 to about 250° C. In an exemplaryembodiment, the helium exits the high-pressure compressor at a pressureof about 50 to about 60 kg/cm² and a temperature of about 190 to about220° C.

In one embodiment, the helium is then transferred to the cold side of arecuperator. The temperature of the helium exiting the cold side of therecuperator is increased to about 150 to about 300° C.

In one embodiment, the amount of electricity generated by the electricalgenerator is about 8,000 to about 15,000 kilojoules of electricity perkilogram of steam generated. In an exemplary embodiment, the amount ofelectricity generated by the electrical generator is about 9,000 toabout 10,000 kilojoules of electricity per kilogram of steam generated.

In one embodiment, the second loop 200 of the system is in operativecommunication with the first loop 100 and comprises an optional solidoxide electrolyzer cell 202 that is used to electrolyze steam at atemperature of about 700 to about 900° C. to form hydrogen and oxygen.As can be seen from the FIG. 2, the second loop 200 comprises the HRSG112, the superheater 104 and a solid oxide electrolyzer cell 202. Thesolid oxide electrolyzer cell 202 comprises a cathode side and an anodeside. Steam is electrolyzed to generate hydrogen on the cathode side,while oxygen generated may be swept away on the anode side by compressedair obtained from an air compressor (not shown). On the cathode side,the second loop 200 can comprise other devices such as, for example,condensers (not shown) for separating steam from hydrogen, heatexchangers (not shown) for extracting heat from hydrogen and steam topreheat air used to sweep oxygen from the anode side, feed water heatersfor preheating water, or the like. On the anode side, the second loop200 can comprise other devices, such as for example, compressors (notshown) for compressing air to sweep oxygen from the anode side, turbines(not shown) for driving an electrical generator (not shown) that is usedto generate electricity, or the like.

In the second loop 200, water enters the HRSG 112 at a temperature ofabout 25° C. and a pressure less than or equal to the pressure of thehelium exiting the turbine 106. In one embodiment, steam leaves the HRSG112 at a temperature of about 200 to about 400° C. and a pressure and apressure less than or equal to the pressure of the helium exiting theturbine 106. In an exemplary embodiment, steam leaves the HRSG 112 at atemperature of about 250 to about 275° C. and a pressure less than orequal to the pressure of the helium exiting the turbine 106.

In one embodiment, steam exits the steam superheater 104 at atemperature of about 400 to about 900° C. and a pressure less than orequal to the pressure of the helium exiting the turbine 106. In anexemplary embodiment, steam exits the steam superheater 104 at atemperature of about 700 to about 775° C. and a pressure less than orequal to the pressure of the helium exiting the turbine 106

In one embodiment, the hydrogen and oxygen derived from the solid oxideelectrolysis system 200 can be stored in hydrogen and oxygen tanksrespectively for use in a reversible type solid oxide electrolytic cell(not shown) that serves as a fuel battery to generate electricity as theoccasion demands.

The hydrogen obtained from solid oxide electrolyzer cell 202 has apurity of greater than or equal to about 90%, based on the moles of thehydrogen and any impurities present. In one embodiment, the hydrogenobtained has a purity of greater than or equal to about 95%, based onthe moles of the hydrogen and any impurities present. In anotherembodiment, the hydrogen obtained has a purity of greater than or equalto about 98%, based on the moles of the hydrogen and any impuritiespresent. In another embodiment, the hydrogen obtained has a purity ofgreater than or equal to about 99%, based on the moles of the hydrogenand any impurities present. In another embodiment, the hydrogen obtainedhas a purity of greater than or equal to about 99.9%, based on the molesof the hydrogen and any impurities present.

The aforementioned method of generating hydrogen is advantageous in thatthe use of a direct Brayton cycle can be used to produce electricity aswell as low pressure, high temperature steam for a solid oxideelectrolyzer system. The system is self-contained in that no electricityor steam is exported or imported from external devices.

Since the helium is always at higher pressure than the steam at allpoints in the system, the risk of water ingress from the second loop 200into the first loop 100 (that contains only helium) is minimized,especially when compared with systems that employ a Rankine cycle. Thedisclosed balanced system 10 also employs a lower helium mass flow rate,lower helium coolant return temperatures and lower system pressures thandirect Brayton cycle systems that produce only electricity. This permitssimplification of the nuclear reactor design as well as materials ofconstruction used in the nuclear reactor.

The following examples, which are meant to be exemplary, not limiting,illustrate the methods of operation of the system described herein.

EXAMPLE

This numerical example has been performed to demonstrate one exemplarymethod of functioning the system. This example has been conducted todemonstrate the advantages that are available by generating electricityalong with steam according to the disclosed method.

FIG. 3 is a depiction of the system upon which the numerical example wasperformed. FIG. 3 comprises the same elements depicted in the FIG. 2,with the exception of the recuperator 116. Each element in the FIG. 3however, has its inlet and outlet points numbered. Table 1 shows therespective values (at each of the inlet and outlet points) for thehelium pressure, temperature and mass flow rate for an optimized systemthat generates electricity and steam. Table 2 shows the respectivevalues (at each of the inlet and outlet points) for the water/steampressure, temperature and mass flow rate for an optimized system thatgenerates electricity and steam.

TABLE 1 Mass flow rate Point # Pressure (kg/cm²) Temperature (° C.)(kg/second) 1 53.86 850 180 2 53.75 850 180 3 52.67 830.1 180 4 52.57830.1 180 5 52.52 830.1 180 6 7.67 279.7 180 7 7.617 279.7 180 8 7.601279.7 180 9 7.449 274.9 180 10 7.442 274.9 180 11 7.293 232 180 12 7.286232 180 13 7.14 221.5 180 14 6.997 31 180 15 6.983 31 180 16 19.89 210.7180 17 19.75 210.7 180 18 19.71 210.7 180 19 19.51 31 180 20 19.48 31180 21 19.46 31 180 22 55.41 212.8 180 23 55.11 212.8 180 26 55.00 212.8180 27 7.14 221.5 180

TABLE 2 Mass flow rate Point # Pressure (kg/cm²) Temperature (° C.)(kg/second) 40 1.0 25 18.81 41 6.8 25.04 18.81 42 6.6 148.2 18.81 43 6.4161.4 18.81 44 6.2 267.2 18.81 45 6.0 725 18.81

From the Tables 1 and 2, it can be seen that while the mass flow ofhelium is maintained at about 180 kilograms/second in the first loop100, mass flow of steam of about 18.81 kilograms/second can bemaintained in the second loop. The helium flow in the first loop can beused to generate electricity of about 8,000 to about 10,000 kilojoulesper kilogram of steam. From Table 2, it may also be seen that the heliumcan be used to raise the temperature of steam from room temperature (25°C.) to about 725° C., which is sufficient to permit electrolysis ofsteam to obtain hydrogen and oxygen in a solid oxide electrolyzer cell202.

While the invention has been described with reference to exemplaryembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention.

1. A method comprising: heating helium in a nuclear reactor; expandingthe heated helium in a turbine located downstream of the nuclearreactor, wherein the turbine is in operative communication with anelectrical generator; extracting heat from the helium downstream of theturbine; generating steam from water and superheating the steam with theheat extracted from the helium such that pressure of the steam is lessthan the pressure of the helium during said generation and superheating;and generating electricity in the electrical generator.
 2. The method ofclaim 1, further comprising transferring the electricity and the steamto a solid oxide electrolyzer cell to generate hydrogen.
 3. The methodof claim 1, wherein amount of electricity generated is about 8,000 toabout 15,000 kilojoules per kilogram of steam.
 4. The method of claim 1,wherein the extracting of heat from the helium and generating steam fromwater and superheating the steam is conducted in a heat recovery steamgenerator.
 5. The method of claim 1, further comprising transferringhelium from a compressor to the nuclear reactor via a recuperator. 6.The method of claim 2, wherein all of the electricity and steamgenerated is used to generate hydrogen.